1. Field of the Invention
This invention relates to long bulk conveyors and to the cooperative combination of flexibly shiftable belt conveyors and travelling shifting apparatus for lateral shifting of such conveyors by guided distributed travelling curvature, the shifting being to maintain convenient proximity to a progressive excavation of a bulk deposit such as lignite for convenient removal of the excavated bulk by the conveyor. More particularly, this invention relates to a belt conveyor comprising a framework comprising a flexible continuous rail or rails, a sequence of relatively rigid roller-carrier frames, and a set of pivotal, slidable, or rigid connections, the rails and frames being assembled by the connections into a flexibly continuous shiftable and accessible whole, the flexible continuity being largely due to the flexibility of the rails with the connections, and not largely due to the roller-carrier frames. This invention further relates to a travelling hoisting, shifting, and guiding vehicle with an adjustably curving rail-engaging guide roller array wherein the vehicle continuously powers and urges the guide array along the rails, and meanwhile powers, urges, holds, and adjusts the guide array in lateral and vertical position and in attitude, orientation, and curvature so as to shift the conveyor through a good distance by keeping the rails in controlled distributed travelling curvature. This invention also relates to an elongate guide carrier beam assembly for carrying a plurality of rail-engaging roller assemblies into linear and varyingly curvilinear engagement with the conveyor rail or rails, all for carrying and drawing the conveyor laterally by the rails while keeping the rails in safely controlled and guided distributed travelling curvature.
2. Description of the Prior Art
At a very early date in the development of open cast mining of lignite desposits it became apparent that sophisticated and heavy equipment would be necessary to remove the lignite in small chunks and pieces and transport it from the point of mining to a pick-up or storage location in economically feasible quantities. Since the mining apparatus which was developed to meet this goal is complex, bulky and heavy, and is designed to successively remove the lignite deposits farther and farther from the pick-up or storage location, it became necessary to develop successively longer conveyors for transporting the lignite to a point of loading or storage. These conveyor systems were designed in sections to facilitate periodic adjustment of the conveyor line to new positions closer to the lignite deposits. As the piecemeal-shiftable conveyor systems became more complex and greater in length, it became increasingly difficult to move the continuous conveyor sections from one location to another site which is closer to the point of mining of the lignite. Accordingly, special track lifting mechanisms were introduced into open cast lignite sites, and the weight of such apparatus has ranged up to 100 tons, and frequently had to be pulled by locomotives. This equipment was sometimes provided with traversing crawlers for quicker cross-travel between the conveyor lines which had to be shifted, and the cost of the conveyor-shifting operation became increasingly prohibitive as the conveyor lengths increased.
Perhaps the most widely used steerable, non-railbound, shifting device is a machine developed in 1953 by Rheinische Braunkohlen A.G., a German Company which introduced several innovations into the conveyor shifting procedure. A high shifting rate was achieved using the new apparatus, and the machine provided good maneuverability on rough and uneven terrain. Other advantages consisting of low maintenance and space requirements and independence of rail mounting, as well as reduced costs, were also realized. The steerable, non-railbound track shifting machine is small and compact, but requires the use of a bulldozer or tractor to provide the motive force required in the shifting technique. The German shifting apparatus includes a roller unit which is characterized by a welded casing fitted with two sets of rollers, one roller in each pair being fixed and the other pivotable on side arms, the pivotable roller designed to bias against the shiftable conveyor railbulb of one of the rails by an adjusting mechanism. Buffer springs in the apparatus serve to compensate for unequal rail head dimensions, and the device is provided with arms for connection to the tractor or bulldozer. The tractor must be provided with a side boom, which is attached to the top of the shifting apparatus after the apparatus is connected to one of the rails of the shiftable conveyor. The side arms extending from the roller unit are supported against one side of the tractor. The roller unit is designed to traverse one rail of the shiftable conveyor as the tractor is driven forward and at an angle with respect to the shiftable conveyor line. When the roller unit of the shifting apparatus is mounted on one of the rails of the shiftable conveyor, the roller with the rail attached is then raised by the boom mounted on the tractor, until one end of the track sleepers is clear of the ground. The tractor is then moved forward along the conveyor, pulling the conveyor sections in the shiftable conveyor into sequential alignment from a first linear position to a second linear position closer to the lignite deposits. If sliding of the tractor occurs, during the shifting or realignment process the tractor must be steered at a slight angle away from the conveyor string in order to provide the proper tractive force and direction to realign the conveyor sections. The distance over which the rail is drawn sideways in one passage is sometimes called a "shifting step", and is represented to be up to two meters on dry and level ground. However, in the first passage, the designers of the German shifting apparatus recommend that the rail and sleepers be loosened from the original linear position, particularly in the case of frozen ground, and the second pass is then utilized to actually displace the conveyor sections from one position to the other. The designers further emphasize that the successive shifting steps ought to be small, rather than large, in order to minimize damage to the rail and the tractor and to achieve a fair tractor travel speed. Counterweights must also be used on the tractor, especially when the ground is soft, in order to better regulate traction.
In the steerable, non-railbound, German shifting machine, great stress is placed on the shiftable conveyor rail resulting from the shifting, lifting and forward motion of the tractor, to great disadvantage. For shiftable conveyor rails with wider rail gauges, the stress and load increases considerably, particularly if the module sleepers must be pulled from a clay or loam base, which sometimes create a suction effect. Such high loads and stresses frequently cause damage to the conveyor rail, and in many cases require replacement of the rail due to severe rail distortion, which prevents subsequent traversal of the rail by the roller unit of the shifting apparatus. Furthermore, the shifting procedure using this equipment is slow and requires multiple passes in order to be effective, particular under circumstances where the lateral displacement of the shiftable conveyor must be extensive.
Many disadvantages are found in the side-boom tractor with two roller pairs as found in the prior art. Firstly, the lateral force on the rail must be of the same order of magnitude as the vertical force, or even exceed it, since the coefficient of sliding friction between the sleepers and the ground will certainly sometimes exceed 1, and the shifting is obtained by dragging one end of the nearby sleepers or cross-ties across the ground, and both ends of some nearby ones. This by itself is very hard on the rails and connections, but this drag has the additional penalties next discussed. Secondly, the offset resulting from any s-shaped or reflex curve in a structural member depends largely on two causes, namely the curvature, and the length over which the curvature is obtained. As is well known in structural mechanics, bending stress is directly proportional to curvature (i.e., inversely proportional to radius of curvature), and since stress is limited by consideration of the material to some allowable stress, then curvature is likewise limited to some peak allowable. It is therefore desirable to have the peak value obtain over a length. But in the prior art, the curvature will be maximum nowhere but within the roller assembly, and the drag forces aforementioned will diminish the length of curvature obtained, thereby diminishing the shifting step. Thirdly, there being only two roller points, and, two points being insufficient to define a curve, the curvature is not defined nor limited by the design of the machine, but rather by the actions of the operator as governed by information, judgement, and attention, which may vary, unfortunately. Fourthly, since the drag forces are suddenly and unpredictably variable, with only the two roller pairs lifting only one side of the conveyor while dragging the other and while breaking adhesion of the ties to the ground, the maximum offset safely obtainable is likewise unpredictable. There might be some safe limit, but since the limit would only occasionally apply, the temptation would build to take too long a step, and then to cause damage. Fifthly, the lifting of one side of the conveyor while dragging the conveyor introduces torsional stresses into the rails; such stresses are simultaneous with the stresses due to lateral curvature, and must be reckoned into the total and thus reduce the allowable lateral curvature. These stresses also tend to break the connections between the rails and the roller-carrier frames. Sixthly, the lifting effects are maximum at precisely the point where the lateral bending and torsional effects are maximum, and therefore further limit the lateral curvature allowable.
These and other disadvantages of the side-boom apparatus of the prior art find expression in frequent breaking of the rails and connections, and in long periods of down-time during conveyor shifts. More detailed discussion and structural analysis of such shifting structures will further illuminate the prior art, and serve to illuminate this invention as well, and such discussion follows.
In the lateral shifting of flexibly shiftable conveyors, it is necessary to bring a travelling interval of the conveyor into a travelling s-shaped or reflex curve, or into some approximation of such a curve. Analysis and understanding of such structures and their deflection curves is usually best accomplished by a progression of idealizations from a first simplest idealization to other more complicated and accurate approximations such as follow.
For a first illumination and a first approximation, consider some conveyor framework as if it were an initially straight beam brought into horizontal bending; such idealizations are sometimes usefully applied to triangulated trusses using the moment of inertia of the chords alone in beam-theory to approximate stresses, then using the rule-of-thumb that calculation will underestimate deflections by 15%, more or less. Such calculation will show that no useful flexible shifting can be obtained in fully triangulated structures of such proportions as are found in strip mining, since even a hundred tons of lateral force on the rails would defect such structures only a few inches, even with a hundred feet of curvature. Such forces and lengths would destroy the conveyor, rather than bring it into useful curvature. This is, of course, one of the reasons that such structures are not fully triangulated, but rather comprise triangulated panels and rectangular panels in alternating sequence. Nevertheless, useful insight can be obtained from the idealization, as follows. The idealization shows that deflection will depend on the magnitude of the curvature everywhere, not just at one point, and that the greatest offset is obtained in a given length by having the absolute value of the curvature be maximum everywhere. To have curvature be maximum everywhere is to have a pair of equal opposite tangent circular arcs, the radius of curvature for maximum safe offset being determined by the depth of the beam (i.e., the width of the conveyor) and the material of the beam, and only by those things. In the art, the material is rail steel, and the tolerable radius of curvature will be a thousand or so times the beam depth. Now, in the prior art here considered, the reflex curve is obtained by application of a shear at the end of the curve, so the curvature resulting varies linearly rather than remaining constant, i.e., the moment diagram is a pair of antisymmetrically disposed triangles, resembling a skewed bow tie. Engineer's beam theory will show the resultant deflection to be two-thirds of that obtained by the stepped rectangular diagram of constant (i.e. circular) positive and negative curvature; therefore the prior art method stresses the structure to a maximum while obtaining only about two-thirds of the maximum ideal or theoretical offset. The conclusion is tentative, pending a more refined model. The ideal for the beam-like truss is obtained by the application of three moments, a first one at the center of the reflex curve, and two others of opposite sense, each half the magniture of the first, at the two ends. Such a system is in equilibrium, and requires no imposition of shears whatever. No lateral force need be applied.
As a second and closer approximation to the shiftable conveyors of the prior art, it is useful to analyze the conveyor frame as approximately a Vierendeel frame in the horizontal plane, i.e. as a bending member having rails as the two chords and having the sequence of roller-carrier frames as the sequence of posts of the well-known Vierendeel frame, and having the four rail-to-frame connectors at the four corners of each frame as the moment connectors between posts and chords which characterize the Vierendeel frame. In such frames, overall shear is resisted by s-shaped or reflex bending of the chords between the posts, and such reflex bending is by far the largest contributor to the overall deflection in case the posts and moment connectors are stiff, and such is the case in the conveyors of the prior art. Seen otherwise, the principal deflections here are panel deflections or shear deflections; deflections due to overall flexure are a very small part of the whole. Therefore the maximum safe deflection in such conveyors is not found when the overall curvature approximates the s-shaped curve composed of two semicircles, but is obtained by having constant maximum panel shear, the overall curve approaching the cross-section of a terraced lawn of constant step height and constant step width, with the individual step-connecting slopes corresponding to the s-shaped curve of the first approximation. The overall average curvature during maximum safe deflection would approximate a ramp more than it would approach the two semicircles of the first approximation.
Even if a complete degree of rotary freedom were introduced into each of the four rail-to-frame connections at every roller-carrier frame (i.e., by making the connections pinned, rather than fixed as in the prior art), the action of the whole would still correspond to a Vierendeel frame with significant flexibility in the posts, since the points of entry of the rails into the post region would be slanted rather than level. The rails would be in curvature everywhere, with inflection points not only at the midpoints of the open shear panels, but also at the midpoints of the triangulated roller-carrier frames. The curve of the rails would then resemble a tilted corrugation, more than a sequence of terraces. The average overall curvature for maximum safe deflection would still approximate a ramp (albeit a corrugated ramp) more than it would approximate the two semicircles of the first overall approximation. However, the safe allowable deflection would be approximately doubled, as can be seen from strain-energy considerations, thus: Suppose, for simple example, that the open panels and triangulated panels were equal in width. Then the portion of the rails within the two types of panels would be equal in length and curvature, and would be twice that of the rails of the prior art, thus the strain energy will be twice as great, the maximum allowable rail curvature would have remained unchanged and, the shear unchanged. Therefore, the lateral load would be the same at lateral safe deflection, the external work must equal the strain energy and the lateral load must deflect twice as far.